As i type this it has been 81 days since Mistress ordered me locked into a CB3000. i don't know when i will be released. (It has been off for brief periods, the longest because the numbered plastic lock broke when i was away from home, and i didn't have a spare or time to improvise a substitute, about 8 hrs. That was on December 31. It wasn't removed again until February 3.)
As Anna said, the spacing is most important, and basically, that is what you have done by using 2 rings. Just use the shortest possible spacer between the ring and the cage, and use however many more it takes on the "outside" to fill the space to the lock hole. There is also the mental aspect. i don't know if i can pull out, because i have not tried. If i succeeded, and, disobeyed Mistress's order, i would feel compelled to confess, and would likely loose Mistress, as i have disobeyed before, and She made it clear this is my last chance. If i fail to obey, She will no longer waste Her time on me.
i started with the "points of intrigue", the medium length one, installed, as Mistress wanted me to suffer if i became hard. These are intended to prevent pullout, and they will, however i experienced actual penetration of the skin (actual holes that were the shape of the points, and bleeding), so Mistress allowed them to be removed. If i were to try them again, i would go with the shortest points, and keep a closer eye on things.
The burning you feel is due to the skin actually "folding" within the ring. Be sure to apply lube all around the cock and balls where the ring rests. Tight clothing will also contribute to the problem, so keep things loose down there. After i had worn the device for about 47 days, i changed the hinged ring for a solid one. That virtually eliminated the problem, although it does still occur occasionally. You can get a solid ring from the manufacturer, it's sold as a ring for the cb2000, but also fits the cb3000.
i was using a good quality silicone lube, but found it didn't seem to last more than a few hours, so i tried baby oil gel, as recommended. That lasts a long time, and has made a difference in comfort for long term use.
Hope this helps.
